In recent years, the cultural heritage of man has been digitalized because digital signals change little through the ages in comparison with analog signals, and thus are appropriate for storage over a long period of time. However, digital recording apparatuses so far cannot keep data stored for 100 years or more.
For example, it is said that hard disc devices for storing data using magnetism have a life of approximately 30 years. This is because a problem often occurs in the mechanism for making a data reading apparatus approach a disc that is rotating at a high speed.
It is said that optical disc devices for CDs (compact discs), DVDs and Blu-ray discs (registered trademark) have a life of approximately 15 years. This is because a problem often happens in the rotating parts, and in addition, the transmittance of light lowers due to the change in the material for coating the surface of a disc over time.
Thus, digital recording devices have a life of only approximately several tens of years, and sooner or later they will be out of order. Therefore, digital data must be periodically copied in a new recording apparatus for storage over a long period of time. This operation is referred to as migration.
The cost for migration is high, which becomes a large economic burden. For example, it is said that the cost for digitally storing three copies of one two-hour film with 4K digital cinema quality (4096×2160 pixels, 24 frames per second) is ¥1,000,000 or higher per year. Furthermore, it is said that the cost for storing all the fragments created in the manufacturing process of one digital film is ¥20,000,000 or higher per year.
Therefore, a new digital recording device for storage over a longer period of time has been demanded. If digital data can be stored for 1,000 years or more without any problems, migration is not necessary, which makes it possible to leave cultural heritage for the next generation at a lower cost.
Conceptual ideas of a number of wafers on which mask ROMs (read only memories) are formed are layered on top of each other, and the entirety of which is sealed with SiO2, have been proposed as a new digital recording device having a potential of achieving the above (see Non-Patent Document 1).
Semiconductor integrated circuit devices, such as mask ROMs, are manufactured by processing silicon as the main material along with other materials, for example, silicon oxide films, and metals, such as copper. Silicon and silicon oxide are very stable substances and have such a feature that their quality does not change over 1,000 years. Meanwhile, metals are oxidized in an environment of a high temperature and high humidity, which causes rusting or corrosion.
In semiconductor integrated circuit devices, metals are used for wiring circuits, and thus are used inside the device covered with a silicon oxide film or a silicon nitride film. Metals are exposed to the outside of the device only through an opening in the silicon oxide film or the silicon nitride film, which is referred to as a pad, for the connection with wires outside of the device for the power supply and data communication (wire bonding). In the case where moisture enters into the device through this opening, the internal metal wires corrode and sooner or later causes a problem.
Digital data is recorded in a mask ROM as a pattern of contacts for the connection between metal wire layers using a mask for lithography during the manufacturing process for the device. As compared to the systems for writing data using a charge or magnetism, the mask ROM, in accordance with the contact method, is strong against a change in the environment or noise in terms of radiation (cosmic rays) and electromagnetism (geomagnetism) and has such features that data can be stored over a long period of time as long as the metal wires do not corrode.
In the case where a mask ROM is sealed with a silicon oxide film or a silicon nitride film without an opening for a pad as proposed above, moisture can be prevented from entering into the device from the outside, and therefore it is expected that the life of the chip can be made to be 1,000 years or more.
FIG. 13 is a graph illustrating the dependency of the life of chips on humidity in accordance with the report of research concerning the life of chips (see Non-Patent Document 2). As shown in FIG. 13, in the case where the humidity inside a chip can be made to be 2% or less, the life of the chip is 1,000 years or more even when the temperature of the chip is 100° C.
From the point of view of the manufacturing cost, it is expensive to fabricate a mask to be used only once in order to write data, such as of a film, in this mask ROM. There is a technology for directly irradiating the resist on a wafer with an electron beam using an apparatus for direct drawing with an electron beam (EB), which substitutes a mask. The use of this technology makes it possible to write data into a mask ROM at a low cost.
As of 2009, a 4-gigabit mask ROM can be manufactured in a 1 cm square chip at a cost of approximately ¥200 by using a 45 nm CMOS technology. Accordingly, a 2.5-terabit mask ROM can be manufactured at approximately ¥140,000 by layering four 15-inch (approximately 38 cm) wafers on top of each other.
In ten years, the integration density of semiconductor integrated circuits will be 100 times greater, and thus the cost per bit could be expected to be 1/100. Accordingly, it is predicted that in 2020, 6-terabit digital data could be stored in one 4.75-inch (approximately 12 cm) wafer, which is the same size as DVDs, at a manufacturing cost of ¥1500.
Therefore, the remaining technical issues are how power is supplied to a sealed mask ROM and how the data can be read out. A technology, which is referred to as a wireless tag (RFID: radio frequency identification), is used as the technology for reading out data wirelessly while supplying power wirelessly.
This is an automatic recognition system for reading out or writing in individual pieces of information on people or things recorded in a medium, which is referred to as IC tag, through wireless communication (retrieving, registering, deleting and updating data), which is used for electronic money cards and electronic tickets, for example. The technology in the above-described Non-Patent Document 1 is also assumed to use such a system.